This talk focuses on the study of two urban topographical typologies and their geometrical variations aiming to understand the role architectural features play in the local urban airflow characteristics and street ventilation. Thermal pollution and chemical pollutant concentrations peak in cities, as opposed to the countryside, due to the high and localized anthropogenic emissions, as well as to the topographical and surface materials properties of the urban fabric. However our understanding of the influence of urban geometry on urban ventilation still remains limited. Most of the previous numerical and wind tunnel studies developed to model urban flow and pollutant dispersion have been devoted to the exploration of idealized building configurations, i.e. buildings of rectangular shapes and flat roofs. The influence of more complex building geometries, such as variable building heights or variable roof geometries, have more recently been researched but only to a limited extent. Furthermore the influence of smaller roughness scale geometrical features, such as the presence of balconies or chamfered building corners on urban flow statistics have not yet been studied in detail. Therefore the understanding as to which extent small roughness scale elements influence the street level airflow characteristics is yet to be fully developed. While urban structures differ among different cities as well as between different neighborhoods within the same city, arguably the urban street canyon and the urban courtyard are amongst the most generalized urban typologies. These urban typologies however can be subject to various architectural formalizations, where roof or façade configurations can adopt different topographies. In order to understand the role these architectural features play in the street ventilation potential, Large-Eddy Simulations (LES) using the FLUENT code have been performed under neutral stability conditions for 8 different architectural configurations. Five street canyon geometrical variations: i) flat roof, ii) pitched roof, iii) round roof, iv) terraced building and v) building with balconies have been studied, together with three courtyard building variations: i) courtyard building with flat roof, ii) courtyard building with chamfered corners, iii) courtyard building with variable cornice profiles. For LES code validation purposes, wind tunnel experiments were also conducted for the first three geometries of the street canyon typology, the flat, pitched and round roofs. The simulation and experimental setups were closely matched and both featured configurations consisting of seven building arrays. The Air Exchange Rate (ACH) between the street canyons and courtyards and the free atmosphere has been computed for the different cases. The results obtained for the studied typologies show that the ACH is very sensitive to the building geometries; therefore, it appears reasonable to suggest that buildings can be shaped to promote urban ventilation. In-canyon vortex dynamics and over-canopy flow conditions, are strongly dependent on the geometric features of the buildings. Pitched and round roof geometries increase in-canyon mean and turbulent velocities, as well as the depth of the shear layer. Three-dimensional cornice configurations for the courtyard typology are shown to generate higher exchanges both for the streets in between the courtyard blocks as well as for the courtyards. The findings provide novel insight into the sensitivity of the flow and turbulence fields to urban morphology. They also underline the influence of three dimensional building geometries as well as small-scale features such as balconies, which were often ignored in prior literature.
